Flame quality and presence monitor for multiburner furnaces

ABSTRACT

Flame-monitoring equipment for indicating the state of a burner flame in the presence of other burner flames is arranged to respond to a range of frequencies higher than those normally utilized in flame-monitoring equipment and which are confined to the root portion of the burner flame. The equipment provides a measure of the flame quality and has a light sensitive element which is arranged to view the root portion of the burner flame and to feed signals to the associated electrical circuitry which determines the flame quality from the number, rather than the amplitude, of the high frequencies present in the electrical output of the element and lying within said range.

United States Patent 72] Inventor John A. Bloice London, England 211 Appl. No. 797,896 [22] Filed Feb. 10, 1969 [45] Patented Oct. 12, 1971 [73] Assignee Memco Limited Isleworth, Middlesex, England [32] Priority Feb. 22, 1968 [3 3] Great Britain [31] 8634/68 [54] FLAME QUALITY AND PRESENCE MONITOR FOR MULTIBURNER FURNACES 10 Claims, 3 Drawing Figs.

[52] US. Cl 340/228.2, 250/206, 328/6, 340/228, 356/218 [51] Int. Cl G01] 1/42, GOlj 5/32,G08b 17/06 [50] Field of Search 340/228.2, 228 S, 228, 227, 409, 210, 214; 356/217, 218,

221, 223; 328/2, 6; 250/833 UV, 83.6 P, 200, 206

[56] References Cited UNITED STATES PATENTS 2,304,641 12/1942 Jones 340/228.2 X

Primary Examiner.lohn W. Caldwell Assistant Examiner-Scott F. Partridge Atrorney- Holman & Stern ABSTRACT: Flame-monitoring equipment for indicating the state of a burner flame in the presence of other burner flames is arranged to respond to a range of frequencies higher than those normally utilized in flame-monitoring equipment and which are confined to the root portion of the burner flame. The equipment provides a measure of the flame quality and has a light sensitive element which is arranged to view the root portion of the burner flame and to feed signals to the associated electrical circuitry which determines the flame quality from the number, rather than the amplitude, of the high frequencies present in the electrical output of the element and lying within said range.

SHEET 2 BF 3 PATENTEDUBT 12 I97! m z z/vmp JOHN ANTHwY aLolcs 8), M f amma/5 FLAME QUALITY AND PRESENCE MONITOR FOR MULTIBURNER FURNACES This invention relates to flame-monitoring equipment for use with multibumer furnaces and more specifically is concemed with equipment devised to operate eflectively over a wide range of burner flame intensities and with negligible risk of the equipment failing to respond to the extinction of a particular flame as a result of the masking effect of neighboring flames.

There is an increasing demand in modern steam-generating apparatus and similar apparatus for the flames of burners feeding a furnace to be individually monitored so that an immediate warning is given should one of the flames falter or extinguish itself. Various techniques have been devised for monitoring individual flames of burners but they all suffer from practical disadvantages and none of them, as far as the applicants are aware, are capable of operating without risk of paralysis over a wide range of flame intensities or for monitoring the flames of a furnace having oppositely directed burners. The use of oppositely directed burners in furnaces is now becoming more common and is a combustion technique in which the burners are arranged in opposed pairs and direct fuel towards one another so that an extremely hot fireball is formed between the pairs by the coalesced flame fronts.

One of the more successful forms of flame monitoring equipment uses a photosensitive element which views the flame front of a particular flame being monitored and provides an electrical output signal having an alternating component whose magnitude is dependent upon the quality of the flame. This system relies on the fact that a burner flame front has a substantial low frequency alternating component of its light output of the order of 100cycles per second or less (hereinafter referred to as the 100 c.p.s. component for convenience), whose presence and magnitude may be used to signify, respectively, whether a burner is alight and the quality of (the flame produced by it. Although light frequencies other than 100 c.p.s. are known to occur to lesser extents in the burner flame front, the component frequencies of the order of 100 c.p.s., apart from the flame-fluctuation frequency which is nonnally about c.p.s., predominate and are naturally used to provide a measure of the flame quality of the burner. However, the relative magnitude of the 100 c.p.s. flame-fluctuation signal is large as compared with the 100 c.p.s. component present in the flame front so that there is a strong tendency for swamping of the photocell output by the constant value components and the 10 c.p.s. signal when monitoring high-temperature flames, with consequent difficulties in detecting the relatively weak 100 c.p.s. signal.

Although the above'described flame-monitoring equipment is widely used nowadays it has certain limitations which make it virtually unusable with opposed burners and seriously impair its usefulness in monitoring one of a multiplicity of closely spaced burners. One of these limitations stems from the fact that the burner flame being monitored may well have a relatively large range of desired operating temperatures. The light output fed to the photosensitive element is not only that of the flame but also that of the often-incandescent refractory material of the furnace lining viewed through the flame, as well as portions of adjacent flames.

In practice, there is towards the high-temperature range of burner operation a high level of background illumination on the photosensitive element which reduces the relative strength of the 100 c.p.s. alternating component of the photocell output signal. For example, perhaps 99 percent of the light falling on a photocell emanates from sources other than the 100 c.p.s. component. The internal resistance of the photocell is naturally reduced by the high level of background illumination so that fluctuations in the internal resistance produced by the 100 c.p.s. component provide relatively small electrical output signals. Also, the load of the photocell, being constant, means that there is always a mismatch between the photocell and the load at one or other ends of the range of burner operation. For example, if the burner is required to be monitored accurately at low-firing rates, or through a dirty window, then the photocell must be able to provide a good signal when its resistance is relatively high as a result of the low light intensity falling upon it. For this to occur the load connected to the photocell must have a high resistance. If the burner is operating at maximum firing rate then the light incident on the photocell is of high intensity so that its resistance is relatively small compared with the load resistance, and the mismatch between the photocell and load resistance results in a considerable loss of useful signal level derived from the c.p.s. component.

A second limitation stems from the fact that although the detection of the 100 c.p.s. component.

A second limitation stems from the fact that although the detection of the 100 c.p.s. alternating component of a flame front is generally considered to be sufficient to distinguish one flame from a neighboring flame, this does not apply when the burners are oppositely directed as for practical reasons it is not possible to view the flame front of a burner flame without also viewing the flame front and thus the 100 c.p.s. component of the oppositely directed burner. The conventional system is thus quite incapable of distinguishing the 100 c.p.s. component of one burner flame which is not operating correctly, from the 100 c.p.s. component present in the flame front of the opposed burner which may be functioning correctly.

A third limitation of the above-described equipment is that for burners required to operate over a wide range of firing rates, the difference in light intensities falling on the photocell is commonly such that unless compensating measures are taken by reducing the light on the photocell at the upper end of the range of temperatures, paralysis of the equipment can occur. Although automatically operated diaphragms have been proposed for cutting down the light incident on the photocell at higher temperatures, they have the same effect as dirt on the window through which the photocell views the flame. The position can arise, therefore, that deposition on the window is interpreted as a poor quality flame. To remove this disadvantage complex systems have been devised for controlling the diaphragm position in accordance with the background radiation which produces a unidirectional output from the photocell. However, such arrangements increase the cost and complexity of the flame monitoring equipment.

An object of this this invention is the provision of improved flame-monitoring equipment.

In accordance with the broadest aspect of the invention there is provided flame-monitoring equipment for indicating the state of a burner flame in the presence of other burner flames by responding to the range of relatively high pulsation frequencies (as herein defined) present in the light output of the root portion of the flame to be monitored, such equipment including a light-sensitive element for viewing the flame root to provide an electrical signal containing components at said frequencies, and electrical circuitry receiving said components and providing an output whose magnitude is a function of the frequencies present in the electrical signal and is a measure of the flame quality.

In accordance with a second aspect of this invention flamemonitoring equipment includes a light-sensitive circuit containing a photosensitive element for providing an electrical output signal containing alternating frequency components corresponding to pulsation frequencies of light incident on the element from a root portion of a burner flame to be monitored; a processing circuit connected to receive the alternating frequency components from the light-sensitive circuit and to amplify those above a predetermined range of lower frequencies to substantially the same amplitude, irrespective of their relative amplitudes above a predetermined minimum magnitude, to provide an output train of pulses of substantially the same magnitude and generated at a rate dependent on the relatively high pulsation frequencies (as herein defined) of the components above said predetermined range of lower frequencies in the signal input to the processing circuit; and a pulse counting and analogue converter circuit connected to receive the output train of pulses from the processing circuit and to derive from them an electrical flame-measuring signal significant in magnitude of the pulse repetition rate in the train of pulses and which is an indication of the flame quality being monitored.

In accordance with a third aspect of the invention flamemonitoring equipment includes a photosensitive element whose electrical resistance varies with the intensity of light incident thereon, a nonlinear impedance load substantially matched to the element so that its impedance varies in sympathy with the element resistance to provide an output voltage signal across the load which varies as a logarithm, i.e. function, of the load current, a coupling circuit transferring alternating components of the voltage signal to input terminals of high-gain limiting amplifier, preferably an integrated circuit amplifier, a negative feedback loop extending from the output terminals of the amplifier to its input terminals and incorporating a shunt path for removing signals at relatively high pulsation frequencies (as herein defined) and above a prearranged lower range of input signal frequencies so that said lower frequency range of signal components is degeneratively amplified to progressively greater extents with diminishing frequencies, a differentiating and detecting circuit converting the limited output signals from the amplifier into a train of unidirectional pulses, a pulse-counting circuit for receiving the pulses and providing an output signal fluctuating with the rate of pulse generation by the amplifier, a potentiometer adjustable to select a desired proportion of the output signal in order to set the equipment to a predetermined threshold frequency, and an analogue converter circuit operating a flame indicator and controlled by the output signal from the potentiometer.

The underlying principle of the invention is based on the discovery by the applicants that a range of light frequencies considerably above those hitherto used for flame monitoring purposes and through this specification referred to, for convenience, as relatively high pulsation frequencies" is produced in the root portion of the burner flame and can be used to operate flame-monitoring equipment. Moreover, this range of frequencies produced in the root portion increases with increasing quality of the burner flame so that, by counting the frequencies produced, rather than by taking a measurement of their magnitude, an excellent indication of the flame quality can be obtained which is localized to the burner flame being monitored. The lower range of frequencies beneath the relatively high pulsation frequencies and which have formed the basis of prior art flame-monitoring equipment are not used in the applicants invention and they fall within the aforesaid prearranged range of lower frequencies. By using the relatively high pulsation frequencies present in each burner flame root to operate the flame-monitoring equipment associated with the burner, individual burners arranged for opposed firing can be monitored independently and without the risk that frequencies generated by one burner will mask the correct reading of the equipment monitoring invention opposed burner. In practice, the relatively high pulsation frequencies used in the applicants invention range upwardly from frequencies of the order of 1,000 c.p.s. and are suitably above 3,000 c.p.s. upwards, frequencies of 10,000 c.p.s. and above being particularly useful in carrying out the applicants invention as they are found to be very localized to a particular region of the flame root.

In carrying out the second aspect of the invention it is naturally preferable that the load for the photosensitive element should have a logarithmic impedance characteristic substantially matched to the impedance characteristic of the photosensitive element which may be a photosensitive transistor, a photocell or a photoresistor. A suitable form of load comprises a forward biased silicon diode connected to provide across it a voltage which varies logarithmically with the current through it. Preferably the photosensitive element in such an arrangement comprises an NPN phototransistor.

The processing circuit of the second aspect of the invention suitably includes a high-gain amplifier, preferably as integrated circuit amplifier, having low-frequency degenerative characteristics for suppressing the lower range of frequencies, that is to say those beneath about 1,000 c.p.s. A convenient way of obtaining low-frequency suppression is by using a negative feedback loop between the output and input to the amplifier and arranging for a shunt circuit to earth to be effective to prevent the feed back of frequencies above the lower range which is to be suppressed. The amplifier is preferably a limiting amplifier having a high gain so that virtually all input signals are limited to substantially the same value in the output. A detecting circuit following the limiting amplifier may then be used to convert the output from the amplifier into a stream of pulses of substantially the same magnitude and polarity. The pulse repetition rate is a function of the frequencies present in the light incident on the photosensitive element and this, in turn, has been found by the applicants to be indicative of the flame quality.

An advantage of the third aspect of the invention is that by arranging for the load on the photosensitive element to have a characteristic which substantially matches the characteristic of the element, a satisfactory match between the resistance of the element and the impedance of its load can be maintained so that the range of light intensities over which the equipment is capable of operating can be considerably extended as compared with conventional flame-monitoring equipment using a constant load. In this way the flame-monitoring equipment is suitable for use with current burners designed to operate with a very wide range of load, without risk of paralysis.

Preferably the detector circuit of the third aspect of the invention includes a diode arranged between the base of an NPN transistor and a capacitor. The capacitor and the diode serve to provide the base of an NPN transistor connected into an emitter follower circuit, with a bias which fluctuates with the pulse repetition rate in the output of the amplifier. The load of the emitter follower circuit suitably takes the form of a potentiometer across which a voltage is generated fluctuating in accordance with the pulse repetition rate. A proportion of this voltage may be applied through a delay circuit to the gate of a silicon-controlled rectifier to switch the current through a relay and flame indicator lamp. The advantage of such an arrangement is that the point of operation of the relay is determined by the setting of the potentiometer of the emitter follower circuit. The voltage tapped off the potentiometer is a function of the frequencies incident on the photosensitive element and thus the point at which the relay operates in relation to the pulse repetition rate can be determined by the potentiometer setting.

Conveniently the equipment is arranged to provide an output signal if the intensity of light on the photosensitive element is less than a predetermined valve. This may be effected by providing the photosensitive element with two power supplies one of which provides a low current at a unidirectional steady voltage to the element and the other of which provides a larger current at a unidirectional voltage on which a ripple frequency is superimposed. A switching means is adapted selectively to connect the power supplies to the element in accordance with the current passing through it. A frequency sensitive circuit which responds to the ripple frequency may be suitably connected to a supply terminal of the element so as to respond to operation of the switching means by providing an output signal signifying that the light intensity on the photosensitive element has fallen beneath a predetermined minimum. In this way the equipment can provide an indication that the sight glass or transparent screen, through which the photosensitive element views the flame, needs cleaning. The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of a portion of a furnace having a burner the flame of which is to be monitored by flamemonitoring equipment;

FIG. 2 is a circuit diagram of the monitoring equipment; and,

FIG. 3 shows a brightness circuit providing an analogue output.

Referring to FIG. 1 a furnace partially shown at 1 has a number of burners arranged for opposed firing and one of which is shown at 2. The burner 2 is arranged in a port 3 formed in a wall 4 of the furnace. Also associated with the port 3 is a sighting window 5 through which an optical arrangement 9 incorporating a phototransistor 6 is able to view a segment of a root portion 7 of a burner flame. The line of sight of the arrangement 9 is shown diagrammatically and it passes across the edge of an impeller plate 8 surrounding the discharge end portion of the burner barrel. The transistor forms part of an electrical circuit diagrammatically illustrated by the box 11 and which provides outputs significant of the quality of the flame and also the cleanliness of the sighting window 5 which can be impaired by dust deposition on its optical components.

The circuit 11 of the equipment is shown in more detail in F IG. 2 and it receives its power from a center tapped secondary winding 12 of a transformer 13. The center tap of the transformer secondary winding is earthed at 14 and a conventional full-wave rectifier l5 and smoothing circuit 16 are arranged to provide a volt supply line on which is superimposed a 1 volt l00 c.p.s. ripple obtained from the full-wave rectifier 15. This supply is connected through two supply circuits 17, 18 to the collector 20 of the photosensitive NPN transistor 6 on which light from the root portion of a flame being monitored is incident.

One of the circuits 17 comprises a simple rectifier 21 providing switching means and having its cathode connected to the collector 20 of the transistor 6 and across which a half volt drop occurs. The supply fed to the collector of the transistor via the rectifier 21 has superimposed upon it the 100 c.p.s. ripple. The other supply circuit 18 to the collector is ar' ranged in parallel with the rectifier and comprises three serially connected resistors 22, 23, 24 of values, respectively of 47 ohms, 220 ohms and a preset 100 kilohm. The junction of the 220 ohm resistor 23 and the I00 kilohm resistor 24 is connected through a smoothing capacitor 25 of 320 microfarads to earth so that the 100 c.p.s. ripple does not appear on the transistor collector when the second supply circuit 18 formed by the resistors is in use. However, the 100 kilohm resistor in the circuit results in the current it is capable of providing to the collector 20 being very limited and if the light intensity falling on the photosensitive transistor 6 is such that the junction of the l00 kilohm resistor and the transistor collector 20 falls beneath 9.5 volts, the rectifier 21 in theother supply circuit 17 is forwardly biased and supplies the transistor collector 20 with current drawn from the 10 volt supply having the 100 c.p.s. ripple on it.

The transistor emitter 26 is connected to earth through a load circuit provided by a silicon diode 27 so connected by forward biasing that the voltage drop across it is proportional to the logarithm of the current through it.

Alternating current components of the voltage signal across the silicon diode 27 are fed through a coupling capacitor 28 of 0 .0l microfarads in series with a 4.7 kilohm resistor 30 to an input terminal 31 of a high-gain integrated circuit amplifier 32 connected to have low-frequency degenerative characteristics and to operate as a limiter. The input terminal 31 of the amplifier is connected to earth through a l,000 picofarad capacitor 33 providing protection from possible sources of highfrequency interference such as a powerful radio transmission. The input terminal 31 of the amplifier 32 is also connected to the output terminal 33a through a negative feedback T-filter network 34 composed of two serially connected resistors 35 each of 150 kilohms having their common junction connected to earth through a 0.33 microfarad capacitor 37 providing a shunt for relatively high pulsation frequencies as abovedefined. In consequence, lower frequency signals produced at the output terminal 33a of the amplifier 32 are fed back negatively to the input terminal 31 to provide the amplifier 32 with degenerative low frequency characteristics whereas a large proportion of the relatively high pulsation frequency signals which enter the feedback network 34 is bypassed to earth through the capacitor 37. The integrated circuit amplifier 32 receives a positive voltage supply from the junction of the 220 ohm resistor 23 and the 47 ohm resistor 22 in the aforesaid supply circuit 18, the other side of the integrated circuit being connected to earth.

The high-gain characteristics of the integrated circuit amplifier 32 together with the degenerative low frequency feedback loop results in the amplifier selectively amplifying the highpulsation frequencies above a predetermined lower range of frequencies and subsequently limiting them so that output pulse signals of substantially the same amplitude are provided at the terminal 330 irrespective to the relative magnitudes of the input signal frequencies of the amplifier 32 provided that such input signal frequencies lie above a predetermined threshold value. The output terminal 33a of the amplifier is connected through a 3.3 kilohm load 40 across which the limited pulses are produced.

The stream of limited pulses from the load 40 are fed through a 3,300 picofarad capacitor 41 to the base of an NPN transistor 42 and also to the cathode of a detector rectifier 43 having its anode connected to earth. The base of the transistor is thus biased positively by an amount which varies with the pulse repetition rate at the output terminal 33a of the amplifier 32.

The transistor 42 provides at its emitter a current which fluctuates in accordance with the base potential. The emitter of the transistor is connected through a 5 ki kilohm potentiometer load 44 so that the transistor 42 behaves as an emitter follower with a 5 kilohm The collector of the transistor is also connected to the 10 volt supply line through a l kilohm resistor 45 across which meter test points 46 are arranged.

The voltage tapped off from the wiper of the potentiometer 44 is fed through the arms of a delay T-circuit 47 formed by two 1 kilohm resistors 48 having their common junction connected to earth through a 2,500 microfarad condenser 50. The other end of the delay circuit is connected to the gate of a silicon-controlled rectifier 51 having its cathode connected to earth and its anode connected through a relay operating coil 52 to one side of the secondary winding 12 of the supply transformer 13. The relay 52 is shunted by a flame indicator lamp 53 and also by a 125 microfarad capacitor 54 providing a holdon circuit for the relay 52 during the half cycles when the silicon-controlled rectifier 51 is not conducting.

The flame monitoring equipment is provided with a flamebrightness indicator circuit shown within the broken outline box 60 in order to warn an operator that the light level incident on the photosensitive transistor 6 is lower than a prearranged value determined by the setting of the kilohm preset resistor 24 in tlie resistive supply line 18 to the collector 20 of the photosensitive transistor 6. During starting up of the furnace the brightness indication will obviously be low. However, if the furnace is functioning normally and the brightness indication is low it may indicate that the sighting window 5 of FIG. 1 and through which the photosensitive transistor 6 views the flame root portion 7 should be cleaned.

The brightness indicating circuit is connected between the collector 20 of the photosensitive transistor 6 and earth. It comprises a l microfarad coupling condenser 61 through which the 100 c.p.s. ripple present at the photosensitive transistor collector is fed to the base 62 of an NPN transistor 63 having its collector connected through a 3.3 kilohm load 64 to the same positive supply terminal as the integrated circuit amplifier. The working point of the transistor is stabilized by a 220 kilohm resistor 65 connected between its collector and base.

The output signal from the collector of the transistor is fed through a l microfarad coupling capacitor 66 to a junction between a pair of rectifiers 67, 68 one of which has its anode connected to earth and the other via a 33 kilohm detector load resistor 70 and 250 microfarad capacitor 71 to earth. The fluctuating voltage developed across the detector load resistor 70 is fed via a second 33 kilohm resistor 72 to the base of a second NPN transistor 73 the emitter of which is connected through a small magnetic flag type indicator 74 to earth while its collector is connected to the same power supply as feeds the integrated circuit.

The above-described equipment operates in the following manner. During normal operation the overall intensity of the light on the phototransistor 6 results in it drawing too much current for the smooth resistive supply circuit 18 to provide, so that the second supply circuit 17 containing the rectifier 21 functions to feed the collector of the photosensitive transistor 6 with a unidirectional voltage supply on which is superimposed a 100 c.p.s. ripple. This ripple at the transistor collector 20 is fed through the coupling capacitor 61 to the brightness circuit 60 and is amplified in the first transistor 63 to provide a signal which is rectified by rectifier 68, and smoothed before being applied as a positive bias to the base of the second transistor 73 which therefore conducts to operate the mag netic indicator 74. This is true whether or not the burner 2 being viewed is alight, provided the furnace is running and the viewing window 5 is not too dirty.

The high-frequency variations in current flowing through the photosensitive transistor 6 as a result of the high-frequency light components present in the flame root portion 7 also flow through the silicon-diode load 27 which provides a voltage signal, significant of the logarithm of the current in transistor 6, to the input terminal 31 of the integrated circuit amplifier 32. The aforesaid lower range of frequencies at the input terminal 31 are suppressed as a result of the degenerative action of the feedback loop whereas the range of relatively high pulsation frequency signals which range upwards from 10,000 c.p.s. are transmitted through the amplifier 32 and are amplified and limited as a result of its high gain. The amplifier output signal pulses are differentiated by resistor 40 and capacitor 41 and are detected by rectifier 43 to provide a bias on the base of the transistor 42 connected as an emitter follower, so as to vary the current through the potentiometer 44 in its emitter circuit. The voltage tapped from the potentiometer 44 is a function of the pulsating frequencies present in the light incident on the phototransistor 6 and is fed through the time delay circuit 47 to the gate of the silicon-controlled rectifier 51 to fire it in accordance with the presetting of the potentiometer 44 and the voltage tapped off it.

If the voltage applied to the gate of the silicon-controlled rectifier 51 falls below a certain value (typically 0.7 volts) the silicon-controlled rectifier 51 will fail to conduct resulting in the relay 52 open circuiting its contacts to sound an alarm or carry out some other duty which is to be performed if the highfrequency content of the root portion of the burner flame being monitored falls beneath a preset value determined by the potentiometer 44 setting.

If, during use, the sighting window 5 through which the flame root portion 7 is viewed becomes obscured, the light incident on the photosensitive transistor 6 diminishes so that the current through it diminishes also. If the current falls to a value such that it can be provided by the resistive supply circuit 18 in preference to the parallel supply circuit 17 containing the rectifier 21, this occurring when the transistor collector voltage rises above 9.5 volts, the rectifier 21 ceases to conduct and the collector receives its current, instead, from the smoothed supply circuit 18. In consequence the 100 c.p.s. ripple is absent and the l microfarad blocking capacitor 61 at the input end of the brightness circuit 60 removes the input signal from it so causing the brightness indicator 74 to operate.

One of the advantages of the above-described circuit is that it is virtually impossible to overload and therefore it is possible to observe the very high frequency components in the flame which otherwise might not be observed because of the masking effect produced by the general background glare within the furnace. This stems from the substantial match maintained between the current-impedance characteristic of light-sensitive element 6 and the similar characteristic of the forwardly biased silicon-controlled load rectifier 27 together with its logarithmic characteristic.

It will be noticed that in the circuit described, only a trip signal is derived from the brightness amplitude, the trip signal occurring when the current through the phototransistor 6 has fallen to such a low value that it can be drawn from the smoothed supply 18 from which the c.p.s. ripple is'absent. If the brightness analogue is required it may be obtained by reconnecting the brightness AC amplifier in the following way described with reference to FIG. 3.

FIG. 3 shows a mains transformer 100 provided with two secondary windings 101, 102. The first secondary winding 101 is connected across a bridge rectifier circuit 103 providing full-wave rectification between its positive and negative output terminals. The unidirectional current output from the bridge contains a 100 c.p.s. ripple component assuming a 50 c.p.s. mains, which is smoothed by a 'rr-filter 104 formed by a resistor 105 capacitively coupled at opposite ends to the negative terminal of the bridge. A continuous light load is maintained on the bridge by a high-value resistor 106 connected between the bridge output terminals to ensure that a minimum current of a few microamperes always flows in the bridge circuit. This current provides a voltage drop across one pair 107 of alternately conducting rectifiers of the bridge 103 of at least several hundred millivolts. The smoothed output voltage from the bridge is applied to a variable load formed by a phototransistor 108 on which light from the furnace is incident and having its emitter 110 connected in series with a forwardly biased silicon-controlled rectifier 111 selected to have a logarithmic current impedance characteristic which matches the characteristic of the phototransistor 108 for reasons already stated in the above-described embodiment. A signal for a flame failure circuit 60' as above described is taken from the emitter 110 of the phototransistor 108.

Connected across the alternating terminals of the bridge are a reference pair of similar rectifiers 112 connected anode-toanode with the junction connected through a bias resistor 113 to the negative terminal of the bridge circuit. The bias resistor 1 13 has developed across it a voltage proportional to the voltage difference between the volt drop across each of the reference pair of rectifiers 112 and the volt drop across each of said one pair 107 of rectifiers of the bridge. The rectifiers and their loads are so arranged that when there is no light incident on the phototransistor 108 the voltage drop across the bias resistor 113 is zero, but increasing illumination of the phototransistor 108 increases the current through the bridge circuit 103 so that the volt drop across its component rectifiers 107 is increased.

The voltage signal developed across the bias resistor 113 with increasing illumination is at a frequency of 100 c.p.s. High parasitic frequencies which may be developed in the secondary winding 101 of the transformer are decoupled by a capacitor 104 shunting the bias resistor.

The signal generated across the bias resistor 113 is fed through a coupling capacitor 115 to the base of an NPN transistor amplifier 116. The transistor amplifier 116 is energized by a smoothed unidirectional current obtained from the second secondary winding 102 of the transformer by way of a smoothing and rectifying circuit 117. The collector of transistor 116 is connected through a load resistor 120 to one end of the second winding 102 whose other end is connected with the emitter of the transistor 116 to earth. The transistor amplifier 116 is required to amplify alternating signals at 100 c.p.s. and is provided with a high degree of degenerative feedback by a resistor 122 connected between its collector and base. The transistor amplifier is thus very stable indeed and amplifies the base signals which increase logarithmically with increase in intensity of illumination of the photo transistor 108.

The output from the transistor amplifier 116 is fed to the base of an instrument driver amplifier 123, by way of a preset resistor 124 providing a sensitivity control and in series with a coupling capacitor 125. The instrument driver 123 is also an NPN transistor having its collector connected to the smoothed positive supply from the transformer secondary winding 102, and its emitter connected to earth through a decoupling capacitor 126 shunting a milliammeter instrument 127 serially connected with a current limiting resistor 128. The decoupling capacitor 126 prevents rapid fluctuations of the instrument.

The base of the driver transistor 123 is connected to the cathode of the rectifier 130 having its anode earthed and providing a detector circuit.

In operation of the above-described circuit variations in brightness of light incident on the photosensitive transistor 108 vary the amplitude modulation of the 100 c.p.s. signal across the bias resistor 113 as previously mentioned. The bias resistor voltage is amplified in the alternating current amplifier transistor 116 and then detected by rectifier 130 and fed to the base of the instrument driver amplifier 123. The reading of the milliammeter 127 associated with the instrument amplifier 123 therefore provides a measure of the light-intensity incident on the photo transistor 108.

It will also be appreciated that the circuit may be adapted to respond to any particular wavelength of light emitted from the burner flame. For example, by using filters, the ultraviolet light content of the flame may be projected onto the phototransistor.

I claim:

1. Flame-monitoring equipment comprising a light-sensitive element, means directing an image of a peripheral portion of a burner flame root onto said element, an electrical load through which current from said element varying with the intensity of light falling thereon passes, said load having an impedance-current characteristic similar to said element; circuit means detecting from the number of pulsation frequencies above flicker frequencies in the signal output of said element a measure of the flame quality, and means indicating said measure.

2. Equipment as set forth in claim 1, including a first current supply circuit to said element and an alternative second current supply circuit to said element; back-biasable switching means in said first current supply circuit for transmitting to said element a DC level having a superimposed ripple frequency; resistive means limiting the current through said second supply circuit and capacitive means for removing therefrom said ripple frequency; and a brightness indicating circuit connected to said element and including means responsive to the presence and absence of said ripple frequency to signify the monitored flame brightness being above and beneath, respectively, a predetermined value.

3. Equipment as set forth in claim 1, including first rectifying means feeding said light-sensitive element from a rectified source of alternating current; second rectifying means conducting simultaneously with said first rectifying means and connected to said source; constant load means for said second rectifying means, and means providing a signal significant of the analogue brightness from the volt drops across said first and second rectifying means.

4. Flame-monitoring equipment including a light-sensitive circuit; a photosensitive element in said circuit providing an electrical output signal containing alternating frequency components corresponding to pulsation frequencies of light incident on the element; an optical arrangement for directing onto said element light emanating from a root portion of a burner flame to be monitored; a processing circuit connected to receive the alternating frequency components from said light sensitive circuit; amplifying means in said processing circuit amplifying to the same amplitude those components present which have a frequency above the range of flicker frequencies to provide an output train of pulses; pulse counting and analogue conversion circuitry connected to receive the output train of pulses from the processing circuit; and indicating means deriving a measure of the flame quality from the pulse repetition rate of said pulse train.

5. Equipment as set forth in claim 4, in which said amplifying means comprises an integrated circuit high-gain amplifier having a degenerative feedback network provided with a bypass circuit for bypassing to earth signals in said feedback network above said range of frequencies.

6. Equipment as set forth in claim 5, including two unidirectional current supply circuits to said element, one such supply circuit including a rectifier for transmitting a direct current level on which is superimposed a ripple frequency, and the second supply circuit containing a chain of resistors and ripple suppression circuitry to provide said second current supply circuit with limited current carrying capacity, ripple frequencies present at the element being detected and amplified by an amplifier to provide a bias voltage for a second amplifier controlling with its output a brightness indicating device having two states indicating, respectively, the brightness incident on the element being above or beneath a predetermined threshold value.

7. Equipment as set forth in claim 4, including analogue brightness indicating circuitry comprising a source of alternating current, a full-wave bridge rectifier providing from said source a unidirectional current available at output terminals of the bridge rectifier, a high-value load resistor connected across the output terminals of said bridge rectifier, a second circuit connected across said bridge-rectifier output terminals, and including said light-sensitive element formed by a phototransistor connected in series with a load having a similar impedance-current characteristic as the transistor; a second rectifier circuit connected across said source of alternating current, a biasing resistor connected from a midpoint of said second rectifier circuit to one direct current output terminal of said bridge rectifier, a connection extending from said bias resistor and providing to an amplifier a signal significant of the volt drop difference across rectifiers of said bridge and said second rectifier circuit, and an instrument driver stage fed by a bias provided by the output of said amplifier, said instrument having means representing the analogue brightness.

8. Flame monitoring equipment comprising a phototransistor, a forwardly biased silicon-controlled rectifier connected in series with the current circuit through said transistor and having an impedance-current characteristic similar to said phototransistor, a limiting high-gain amplifier having degenerative characteristics for flicker frequencies and connected across the transistor load, output terminals to said amplifier and connected through a negative feedback loop to said input terminals, a shunt path in said feedback loop removing to earth signals above the flicker frequencies so that said flicker frequencies are suppressed, a differentiating and detecting circuit connected to receive output signals from said amplifier and to provide therefrom a train of pulses, a circuit providing a signal instantaneously significant of the rate at which pulses are received from said differentiating and detecting circuit, a potentiometer adjustable to select a desired proportion of the varying output signal to set the equipment to a predetermined threshold frequency, and an analogue converter circuit operating a flame indicator and controlled by the output signal from the potentiometer,

9. Equipment as set forth in claim 8, including first and second current supply circuits to said phototransistor; a blackbiasable diode in one of said circuits; a resistance chain in the second of said circuits; a full-wave bridge rectifier providing direct current with a superimposed ripple to said circuit; a filter in said second circuit for removing said ripple; a brightness indicator circuit supplying a signal at the frequency of said ripple, from said photo transistor collector to a brightness indicator circuit; an amplifier and detecting circuit providing from said ripple frequency signal a bias voltage which is absent in the absence of said ripple; a driver stage switched between a conducting and a nonconducting condition by the presence and absence, respectively, of said bias; and, a brightness indicator by the output of said driver.

10. Equipment as set forth in claim 8, including a trans former, a secondary winding to said transformer feeding a fullwave bridge rectification circuit and a half-wave rectification circuit in parallel, a bias resistor connected between the center point of said half-wave rectification circuit and a direct current output terminal of the same polarity of the bridge, a resistor connected across direct current output tenninals of the bridge, a filter circuit through which said bridge supplies direct current to said phototransistor and said load, the center ference, a driver amplifier controlled by said bias stage, and a meter operated by said driver stage and providing an output which is an analogue of the brightness of light incident on the phototransistor. 

1. Flame-monitoring equipment comprising a light-sensitive element, means directing an image of a peripheral portion of a burner flame root onto said element, an electrical load through which current from said element varying with the intensity of light falling thereon passes, said load having an impedancecurrent characteristic similar to said element; circuit means detecting from the number of pulsation frequencies above flicker frequencies in the signal output of said element a measure of the flame quality, and means indicating said measure.
 2. Equipment as set forth in claim 1, including a first current supply circuit to said element and an alternative second current supply circuit to said element; back-biasable switching means in said first current supply circuit for transmitting to said element a DC level having a superimposed ripple frequency; resistive means limiting the current through said second supply circuit and capacitive means for removing therefrom said ripple frequency; and a brightness indicating circuit connected to said element and including means responsive to the presence and absence of said ripple frequency to signify the monitored flame brightness being above and beneath, respectively, a predetermined value.
 3. Equipment as set forth in claim 1, including first rectifying means feeding said light-sensitive element from a rectified source of alternating current; second rectifying means conducting simultaneously with said first rectifying means and connected to said source; constant load means for said second rectifying means, and means providing a signal significant of the analogue brightness from the volt drops across said first and second rectifying means.
 4. Flame-monitoring equipment including a light-sensitive circuit; a photosensitive element in said circuit providing an electrical output signal containing alternating frequency components corresponding to pulsation frequencies of light incident on the element; an optical arrangement for directing onto said element light emanating from a root portion of a burner flame to be monitored; a processing circuit connected to receive the alternating frequency components from said light sensitive circuit; amplifying means in said processing circuit amplifying to the same amplitude those components present which have a frequency above the range of flicker frequencieS to provide an output train of pulses; pulse counting and analogue conversion circuitry connected to receive the output train of pulses from the processing circuit; and indicating means deriving a measure of the flame quality from the pulse repetition rate of said pulse train.
 5. Equipment as set forth in claim 4, in which said amplifying means comprises an integrated circuit high-gain amplifier having a degenerative feedback network provided with a bypass circuit for bypassing to earth signals in said feedback network above said range of frequencies.
 6. Equipment as set forth in claim 5, including two unidirectional current supply circuits to said element, one such supply circuit including a rectifier for transmitting a direct current level on which is superimposed a ripple frequency, and the second supply circuit containing a chain of resistors and ripple suppression circuitry to provide said second current supply circuit with limited current carrying capacity, ripple frequencies present at the element being detected and amplified by an amplifier to provide a bias voltage for a second amplifier controlling with its output a brightness indicating device having two states indicating, respectively, the brightness incident on the element being above or beneath a predetermined threshold value.
 7. Equipment as set forth in claim 4, including analogue brightness indicating circuitry comprising a source of alternating current, a full-wave bridge rectifier providing from said source a unidirectional current available at output terminals of the bridge rectifier, a high-value load resistor connected across the output terminals of said bridge rectifier, a second circuit connected across said bridge-rectifier output terminals, and including said light-sensitive element formed by a phototransistor connected in series with a load having a similar impedance-current characteristic as the transistor; a second rectifier circuit connected across said source of alternating current, a biasing resistor connected from a midpoint of said second rectifier circuit to one direct current output terminal of said bridge rectifier, a connection extending from said bias resistor and providing to an amplifier a signal significant of the volt drop difference across rectifiers of said bridge and said second rectifier circuit, and an instrument driver stage fed by a bias provided by the output of said amplifier, said instrument having means representing the analogue brightness.
 8. Flame monitoring equipment comprising a phototransistor, a forwardly biased silicon-controlled rectifier connected in series with the current circuit through said transistor and having an impedance-current characteristic similar to said phototransistor, a limiting high-gain amplifier having degenerative characteristics for flicker frequencies and connected across the transistor load, output terminals to said amplifier and connected through a negative feedback loop to said input terminals, a shunt path in said feedback loop removing to earth signals above the flicker frequencies so that said flicker frequencies are suppressed, a differentiating and detecting circuit connected to receive output signals from said amplifier and to provide therefrom a train of pulses, a circuit providing a signal instantaneously significant of the rate at which pulses are received from said differentiating and detecting circuit, a potentiometer adjustable to select a desired proportion of the varying output signal to set the equipment to a predetermined threshold frequency, and an analogue converter circuit operating a flame indicator and controlled by the output signal from the potentiometer.
 9. Equipment as set forth in claim 8, including first and second current supply circuits to said phototransistor; a black-biasable diode in one of said circuits; a resistance chain in the second of said circuits; a full-wave bridge rectifier providing direct current with a superimposed ripple to said circuit; a filter in said second circuit for remOving said ripple; a brightness indicator circuit supplying a signal at the frequency of said ripple, from said photo transistor collector to a brightness indicator circuit; an amplifier and detecting circuit providing from said ripple frequency signal a bias voltage which is absent in the absence of said ripple; a driver stage switched between a conducting and a nonconducting condition by the presence and absence, respectively, of said bias; and, a brightness indicator by the output of said driver.
 10. Equipment as set forth in claim 8, including a transformer, a secondary winding to said transformer feeding a full-wave bridge rectification circuit and a half-wave rectification circuit in parallel, a bias resistor connected between the center point of said half-wave rectification circuit and a direct current output terminal of the same polarity of the bridge, a resistor connected across direct current output terminals of the bridge, a filter circuit through which said bridge supplies direct current to said phototransistor and said load, the center point of the half-wave rectification circuit being connected to an amplifier transistor for amplifying the difference in volt drop across rectifiers of the half-wave rectification circuit and the bridge, a detecting circuit providing a bias level which fluctuates in accordance with the magnitude of said difference, a driver amplifier controlled by said bias stage, and a meter operated by said driver stage and providing an output which is an analogue of the brightness of light incident on the phototransistor. 